Transom Load Advice Needed

My 67' Larson 166 (Length: 16'4" Beam: 77" Transom: 68.5" wide) has no load plate and I want to know if a heavier motor is an issue after a full gut, reinforcement, and rebuild. Factory catalogue from 67' lists a max HP of 100, but no weight capacities.

Transom:

Factory transom had begun rotting due to poorly sealed holes and some small glass defects where there were delams/air gaps on the top corners. Factory transom was 1/4" chop glass, 3/4" core, sprayed chop, and tabbed with sections of ~24 oz roving one layer thick.

I increased transom thickness and deleted the notch/well for a 25" motor and a narrow transom fish box. I replaced it from the outside using epoxy resin and skinned it with 1/4" of 1708 biaxial. The core is 2 layers of 3/4" marine ply with a layer of 3/4oz csm sandwiched. Inside, it is tabbed with 3 layers of 1708 (4" 6" 8" tape), one layer of 3/4oz csm tabbed 5", and one layer 1708 tabbed 6"

Stringers:

Originally each stringer was a single strip of roving laid in this shape: ¯|_ lengthwise with a plank of wood stuck to the top for affixing the deck. They were replaced as the plank had rotted and due to poor installation, they were horribly misshapen with large wrinkles in the roving leaving gaps between the "stringers" and hull so water could directly contact the foam through the central bilge .

They were cut, ground out, and replaced with a 1.25" laminated plywood stringer. Both sides of each stringer were tabbed in with 3 layers of 1708 and encapsulated in a layer of 1708.

Background info done.

Factory approved motors were 2 strokes, max 100hp ~250-270lbs or the 166 with a 120hp MerCruiser comboard. I want to put a heavier/newer outboard on it ~340-370lbs

I used this rough deflection/sag calc The Sagulator – WoodBin (total, center load) and it seemed to indicate that my improved transom (fiberglass excluded) even with the larger motor would have 1/3 the deflection of the factory spec.

I considered the center of gravity as well and locating my deep cycle close to the bow would completely offset the change if something that small even matters

Coast Guard calc CFR-2002-title33-vol2-sec183-43.pdf (govinfo.gov) puts me at 90hp safely.
The max persons is calculated at 6 people/912lbs. No idea on how to get max persons, motor, gear. And if the weight comes at the penalty of only 5 people that okay since it will only have 3 seats

I got some pushback on another forum, but couldn't get any numbers, data, or resources. Is there something I might be missing that would have people concerned about an extra 120lbs despite the transom reinforcement? I see a lot of folks who seems to know a bit more here since its more for building so hopefully I can get advice.

 

I would not go bigger than the 115, but a 90 would be nice on it and they are still like 350 pounds for a 4 stroke.

The 150 is not gonna work. You have to remember it is an old boat and the hull was also designed for a certain hp; not more. You did not change the hull.

The only thing I don't like is removing the splashwell took away some strength at the upper transom. The intersection can be structural.

And sometimes, knees are also in the picture and structural.

So, we'd need pictures to offer any sound advice.

Also, explain the outer laminate in detail..

 

Sure, the outer skin was removed leaving a 3" lip and ground to a taper. Any gaps were smoothed with thickened epoxy and the whole thing was coated in epoxy. One layer of 3/4oz csm was initially applied stopping 2" from the top to allow the inner skin to wrap over the transom. The next 2 layers of 1708 stopped 4" and 6" from the top for the same reason and to stagger seams as well as being slightly larger than the last covering more of the lip. Before the next layup, after curing, it was ground with 60 grit flapwheel for a mechanical bond.
To do I have the next 3 layers of 1708 will go over the top of the transom onto the top of the fish box 4, 6, and 8 inches.

On the topic of knees and upper bracing the box is going to be glassed to the boat structurally. The top of the fish box is a piece of 17"x 68.5" marine ply that will be tied to the outer skin of the transom as well as the underside having 2-3 layers of 1708 to affix it to the inner skin. It has a centered 2 1/4" x 16" notch mirrored inside the box to allow access to the outboard bolts from above and below as well as directing any water straight into the bilge. The top will be supported by two pieces of 1/2" marine ply 11-17" x 21" spaced 36" apart that will be tabbed to the top, transom and deck with 2-3 layers of 1708. I'm not sure if where they connect to the deck just having the 6lb closed cell foam under is enough support for a "knee" or if I should do additional reinforcement under deck. The bottom of the fish box will have (2) 11-13" x 9" 1/2" plywood supports placed directly over the stringers and glassed in. I wish I had though of doing them as part of the stringers, but hindsight is a b**** lol

*edit* I don't plan on going over 100 actual hp, more a 70/90hp 4 stroke. For a moment I did consider taking the 175 Ocean Pro I was given free and placing a throttle limiter to not exceed 3,500rpm as the weight at 370 didnt seem concerning. I'm deciding to trade it though as its less hassle and don't make the Wa Coast Guard have a literal aneurism thinking I'm unhinged enough to throw all 175 at in like I'm trying to win a darwwin award

 

Additionally I'm considering fully enclosing the gunnels and filling with foam. It would re-stiffen sides, give a cleaner look, as well as stopping hornets and wasps from making nests under there I'd just leave glassed access holes for any rod holders and downriggers. I'm just not sure if I should do it with foam board to glass over or if I should use plywood. It would extend all the way to the transom and I was thinking that it might provide extra bracing, but being only 2-8" from the sides not sure it would do much

 

The Yamaha F70 is lightest in class at about 275 pounds.

It sounds like the exterior work is suitable for 70-90 hp. The transom ought to be tied structurally so the entire length is not deflecting. This can be achieved with the splashwell.

 

Yes, the full length of the transom is tied structurally to the sidewalls of the hull by the top of the fish box

 

Maybe a picture would explain why I'm trying to say better. Its just a dry fit before the bolt access and hatch hole is cut.

 

I don't quite understand what I am looking at...

 

Here is a color coded wire-frame diagram.
Red is the transom
Purple are the stringers
Green is the top
Blue is the framing for the fish box without the front panel so you can see the structure inside.
Black is obviously the holes I need to cut

The top (green) will be glassed to the transoms outer skin on the top, to its inner skin on the underside, and to the hull at the gunnels
The fish box (Blue) will be glassed to the top, to the transom, to the deck, and to the inside of the stringers

 

How are you going to tilt the engine up? Or bolt it on with the fish box ?

 

See that smaller black hole? That goes all the way through the fish box so I can get a wrench onto the nuts for the outboard from above or below.
As far as tilting, the tilt tube is the pivot and forward of the cowling in most cases. Something like this F70 I'd have almost 90 degrees of travel without issue. Like this Evinrude 135 2 stroke I'd only get 45 degrees because the cowling is so far forward. Still clear of the transom, but not as high.

 

Looks good to me.